Image enhancement for improved quality of image through sharpening the image has been widely known. For example, conventional television receivers may utilize a contour compensation technique for enhancing rising-edge/trailing-edge steepness in image signals which represent contours of the images to be displayed. In the contour compensation, high-frequency components are extracted from video signals (brightness signals) before they are inputted to the television receiver display device. The high-frequency components are amplified and then added to the video signal, whereby improved visual quality of the displayed image is achieved through improvement in frequency characteristics of the degraded video signals after various processes performed in a number of circuits from initial input to the receiver to final input to the display device.
Conventional image enhancement techniques including the contour compensation as described above are generally based on linear digital-signal processing, and for this reason none of these techniques can make use of any frequency components which have higher frequencies than the Nyquist frequency, i.e. any frequency components which have higher frequencies than a half of the sampling frequency of a target image. Consequently, it has been impossible to restore frequency components which have higher frequencies than the Nyquist frequency for improved quality of image, or to accomplish sharpening of the image by utilizing such frequency components. As a result, there has been a problem in HDTV television (High Definition Television) for example. Specifically, in the “Full High-Vision” (1080×1920 pixels) television sets, when image resolution of the incoming image signals is lower than that of the HDTV and an image enlarging process is performed in the display device, the resulting images are blurred images. Since these blurred images do not have near-Nyquist frequency components, none of the conventional image enhancement techniques are capable of extracting target frequency components for amplification, and therefore it has been impossible to improve the quality of these images.
In an attempt to solve the problem, different approaches are being taken. One example is an attempt to utilize inter-frame or intra-frame self-correlation, etc. (see Non-patent Document 1 and Patent Document 3 for example) in order to enhance the resolution so that sharp images will be obtained even if image enlarging processes as described above have already been performed. Another example is an attempt to utilize anisotropy diffusion filters (see Non-patent Document 2 for example) in order to smooth the edges in their tangential directions to a certain degree while smoothing the edges in the vertical direction to a certain different degree. Still another example is an image processing device (Patent Document 1) which utilizes nonlinear circuits for adjusting coring amount, clipping amount, enhancing amount, limitation amount, etc. of a signal to be added to the image signal in order to emphasize high-frequency components for improved quality of the image. Also, there is proposed an image quality compensation circuit which utilizes nonlinear conversion circuits (Patent Document 2) for ringing suppression at edge portions of the pulse or step waveforms in the image signals, with an object of compensating for high-frequency signals without sacrificing the quality level of the image.